This UCLA Symposium will address the molecular approaches being used to study ionizing radiation damage to DNA. Although ionizing radiation was the first established mutagen and is a well documented carcinogen, new technologies have only recently led to identification and characterization of complex molecular lesions. This information will accelerate research aimed at understanding the biological consequences. Information on the relationship between chromosome structure and damage repair genes are providing answers to fundamental questions of how ionizing radiation damage is repaired. Definition of the role of ionizing radiation in stress responses, signal transduction and oncogene activation should ultimately lead to a more comprehensive understanding of links between damage production and damage processing in studies on repair, mutation and carcinogenesis. This meeting will provide a forum for interaction between molecular and radiation biologists interested in multidisciplinary approaches to study of basic damage and repair mechanisms and highlight fruitful areas for new research by documenting exciting recent studies aimed at elucidating molecular processing of ionizing radiation damage to DNA.